Semiconductor devices are commonly found in modern electronic products. Semiconductor devices vary in the number and density of electrical components. Semiconductor devices perform a wide range of functions such as analog and digital signal processing, sensors, transmitting and receiving electromagnetic signals, controlling electronic devices, power management, and audio/video signal processing. Discrete semiconductor devices generally contain one type of electrical component, e.g., light emitting diode (LED), small signal transistor, resistor, capacitor, inductor, diodes, rectifiers, thyristors, and power metal-oxide-semiconductor field-effect transistor (MOSFET). Integrated semiconductor devices typically contain hundreds to millions of electrical components. Examples of integrated semiconductor devices include microcontrollers, application specific integrated circuits (ASIC), standard logic, amplifiers, clock management, memory, interface circuits, and other signal processing circuits.
A semiconductor package may include a plurality of stacked semiconductor die. The semiconductor die can be stacked while still in wafer form, i.e., one die in an upper wafer over a corresponding die in the lower wafer. FIG. 1a shows semiconductor wafer 50 positioned over semiconductor wafer 52. Semiconductor die 54 on wafer 50 must be aligned with semiconductor die 56 on wafer 52. The same alignment is required for all semiconductor die on wafers 50 and 52. FIG. 1b shows semiconductor wafer 50 bonded to semiconductor die wafer 52 with semiconductor die 54 aligned with semiconductor die 56. The alignment between all semiconductor die on wafers 50 and 52 is a difficult process during wafer bonding, particularly for thin wafers and different technology wafers, due to breaking, different coefficients of thermal expansion (CTE), process alignment tolerances, pitch tolerances, and maintaining alignment during the bonding and curing. The variance in physical features from one edge of the whole semiconductor wafer to the opposite edge reduces alignment tolerances and tends to cause many defects and low yield. Aligning more than two semiconductor wafers is a particular problem. One solution involves an active alignment using an optical sensor and lens. The relative position of the wafers is optically monitored and feedback is provided to the alignment equipment. The active alignment is a slow and costly for manufacturing.